In recent years, from the point of view of environmental protection, there is a demand for materials that do not emit hazardous substances during utilization as well as disposal and incineration, and are easily decomposed in the soil after having been put to disposal. For such materials, application of molded products consisting of magnesium alloys, which are light and easily decomposed in the soil, has been studied to substitute plastic products heretofore widely utilized.
Heretofore, since these types of magnesium alloys generally have low corrosion resistance, and long-lasting corrosion resistance cannot be achieved even if normal painting is performed, the alloys are normally subjected to chemical treatment, such as chromic acid treatment, to ensure corrosion resistance, and then painted for coloring and weather resistance. As a method that does not rely on chemical treatment, a method for forming an aluminum layer on the surface of a magnesium alloy molded body, and coloring it by painting, is proposed in Japanese Patent Laid-Open No. 9-192261 (1997).
However, according to the above described conventional methods, since a product is provided with corrosion resistance and coloring by painting, or a product is colored by painting, the color depends on the type of paint, so that the paint must be changed depending on a color desired and paint components such as pigments need be varied for use. As a result, since impurities such as pigments disposed of in recycling vary, there was a problem that the removal of impurities becomes complicated.
In addition to the ease of recycling, magnesium alloys have attracted public attention as a casing for devices for their lightness and excellent heat dissipating characteristics. Products in which magnesium alloys are practically used include casings of portable household electrical appliances (note-type personal computers, portable cassette-tape players, portable MD players, etc.), and wheels of vehicles. Magnesium alloys include AZ 91 series for casting and AD 31 series for sheet metals, most of which are used for casting, and are processed by a die-casting method and a thixo·molding method.
The surfaces of some potable household electrical appliances carry logotypes such as brand names. In many appliances, brilliant stickers or the like are bonded to these logotypes. However, brilliant surfaces are made by directly cutting casings to indicate logotypes in many small appliances that must be light, such as portable cassette-tape players and portable MD players. When aluminum-based materials are used as casings, brilliant surfaces can be made without special treatment to the cut surfaces because of the high corrosion resistance of the material. However, since aluminum-based materials are inferior to magnesium alloys in lightness and machinability, the use of magnesium alloys is preferable in casings. On the other hand, since the corrosion resistance of magnesium alloys is poorer when they are used in casings, the cut surfaces must be subjected to an anticorrosion treatment to make brilliant surfaces.
As the methods of anticorrosion treatment for brilliant surfaces, (1) an anodizing method, in which a transparent anodized film is formed and a transparent protective resin film is formed thereon, and (2) a vapor deposition method, in which an anticorrosive metal is vapor-deposited, and a transparent protective resin film is formed thereon, have been proposed and practically used.
However, anticorrosion treatments practically used as described above have the following problems:    (1) The anodizing method requires a complicated jig for electrically contacting desired locations of members to be treated, and a mechanism for checking the electrical contacting state in the anodizing treatment. Further there was a problem that brilliant surfaces formed by this anodizing method have lower brightness, and original brightness of the cut surface was absent.    (2) In the vapor deposition method, although masking can be used for the anticorrosion treatment of a large area, the treatment is very difficult for small and complicated cut surfaces representing brand names or the like.
Therefore, from the point of view of corrosion resistance of magnesium alloy molded products, an object of the present invention is to provide a magnesium alloy molded product that excels in corrosion resistance and is easy to recycle; a magnesium alloy molded product that excels in corrosion resistance, can be colored, and is easy to recycle; and a method for manufacturing such magnesium alloy molded products.
From the point of view of the corrosion resistance and the brilliant surfaces of magnesium alloy molded products, an object of the present invention is to solve the problems in conventional methods, and to provide a magnesium alloy molded product that has a brilliant anticorrosive structure having an anticorrosive protective film without lowering the brightness of the brilliant surface.